This invention relates to magnetorheological fluids, and more particularly, to thickening of non-aqueous, glycol-based magnetorheological fluid formulations.
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. These induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators that use magnetorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, brakes for controllable suspension systems, vibration dampers in controllable power train and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. The MR fluid must be compatible with materials encountered in operation. When used in controllable power train mounts, for example, the MR fluid is in contact with natural or synthetic rubber. Typically, mounts are primarily made of natural rubber, and so the MR fluid must be compatible with, i.e. not soluble in, natural rubber. Mineral oils, synthetic hydrocarbons and esters often used in MR fluids have a tendency to swell natural rubber, thereby degrading its physical properties, which is undesirable. Silicone oils do not swell natural rubber, but are not acceptable in manufacturing environments that include adhesion and painting processes. Therefore, conventional base oils can cause difficulties when used to formulate MR fluids for controlled mounts. The only known MR fluid that is compatible with natural rubber mounts is the fluid disclosed in U.S. Pat. No. 5,670,077, which discloses aqueous MR fluids in which the base liquid is primarily water. In addition to water, the MR fluid further contains a glycol compound. Passive hydramounts often use ethylene glycol and water as the mount fluid because the fluid is compatible with natural rubber and the ethylene glycol provides a wider operating temperature range compared to only water.
Despite their compatibility with natural rubber, ethylene glycol and water cannot be used as the base liquid for an MR fluid because the presence of water causes the iron particles to rust, thereby affecting the magnetic performance over time. In addition, the presence of water, even in small amounts, detrimentally affects the performance of the mount at high temperatures. Thus, it is desirable to provide an MR fluid in which the liquid vehicle is substantially free of water.
Because MR fluids contain noncolloidal solid particles which are at least five times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the liquid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates. Without some means of stabilizing or suspending the solid, sedimentation and/or flow induced separation of the solid phase from the liquid phase will occur. Such separation will have a drastic and detrimental effect on the ability of the MR fluid to provide optimal and repeatable performance. Thus, the magnetizable particles are kept in suspension by dispersing a thixotropic agent or thickener in the liquid vehicle.
While glycols, such as ethylene glycol, may be used by themselves as the base fluid for mount MR fluids, due to their strong polar nature, most thickeners that have been used successfully for non-polar mineral and synthetic oils do not work in a glycol-based base fluid such as ethylene glycol. Many thickeners are recommended for aqueous systems containing small amounts of glycols, but for glycol systems substantially free of water, there are few thickeners that have been recommended. One thickener that has been recommended for use with glycols is polyvinylpyrrolidone (PVP). However, PVP-thickened fluids settle and have an unacceptably high viscosity. There is thus a need for a non-aqueous MR fluid formulation that is compatible in MR devices containing natural rubber, which fluid may be effectively thickened, and which does not experience settling of the magnetizable particles and unacceptably high viscosity during use.
The present invention provides a magnetorheological fluid formulation comprising magnetizable particles dispersed in a non-aqueous liquid vehicle comprising a glycol-based fluid thickened effectively by either a fully treated fumed silica or an organoclay such that less than about 20% particle settling occurs in use. The surface treated fumed silica is fully treated with a treatment agent having a molecular weight of at least about 200, and advantageously, at least about 600, such a polydimethylsiloxane. The organoclay comprises sepiolite and/or palygorskite, and advantageously comprises a clay mixture of about 50-95% sepiolite, palygorskite, or a mixture of sepiolite and palygorskite, the balance smectite, with a surface treatment including reaction with a quaternary ammonium compound. In an exemplary embodiment of the present invention, the glycol-based fluid consists essentially of propylene glycol and optionally ethylene glycol.